Anticorrosive back-fill method



United States Patent 3,192,720 ANTECQRROSZVE BACK-FELL NEETHGD Edward Schaschl and Glenn A. Marsh, Crystal Lake, Ill, assignors to The Pure Oil Company, (Ihicago, lilL, a corporation of flhio No Drawing. Fiied June 5, 1961, Ser. No. 114,676 4 Claims. (Cl. 61-35) This invention relates to anticorrosive back-fill compositions for buried steel structures, or for storage-tank foundations, which materially retard the corrosion rate of such structures or tanks. More particularly, this invention relates to anticorrosive back-fill compositions comprising a homogeneous mixture of about 90 to 99% of sand containing all particle sizes (including fines) that will pass through a 40-mesh sieve and 1% to of 200-mesh, expanding-lattice clay, such as bentonite. The preferred composition of this invention contains 93 to 97% sand, as just defined, with 3 to 7% of 200-mesh bentonite, based on the dry weight of the mixture; more specifically, it has been found that the best results are obtained by using about 3% of 200-mesh bentonite with about 97% of sand containing all particle sizes (including fines) that will pass through a 40-mesh sieve. Another feature of the invention is the inclusion of about 0.01% to 0.1% by weight of an alkali metal sulfite in any of the foregoing compositions.

It is known that the resistance of bentonite to water is increased by using mixtures of bentonite and sand. A seepage-resistant mass can be formed by using layers of fine sand and layers of bentonite next to layers of gravel, or a mixture of equal parts of bentonite and sand, in accordance with the prior art. These compositions are used to prevent the seepage of water through cracks and seams, or around concrete abutments, foundations or dam structures. Also, the deleterious effect of dissolved oxygen on corrosion of metals is known.

Structures, such as pipelines, buried in the earth are usually surrounded or back-filled with soil that has been excavated. Sometimes sand is used as the back-fill mateterial in order to minimize subsequent settling. The bottoms of storage tanks placed in contact with the ground are frequently supported by a cushion of sand in order to permit conformity between the supporting soil and the base of the tank.

Sand, however, is very permeable to water. Normally, water present in the soil is essentially saturated with dissolved oxygen. We have found in experimental work that corrosion rates arising through both long-cell action and local-cell action are extremely high when steel is in contact with moist aerated sand. Many failures of buried pipes and of tank bottoms in contact with sand are attributable to this accelerated corrosion. As an example of corrosion under the conditions of water-saturated sand, let us consider a time immediately after a heavy rain, at which time a buried structure is essentially buried in water. The sand surrounding the structure is watersaturated; all the crevices or interstices between the sand grains which normally are filled with air are now filled with water. At this point, the condition is not especially corrosive. Now as the water percolates through the soil to lower depths, the interstices once more fill with air. At the time that the sand grains adjacent to the buried structure are wet, but air-saturated, corrosion becomes highly accelerated. The oxygen of the air is actually the corrosive ingredient but only in the presence of a film of water.

By rendering the sand impermeable to water, we achieve impermeability to dissolved oxygen as well. In

order to effect this reduction in permeability, we incorporate into the back-fill sand a clay-like material such as bentonite. We utilize 1% to 10% by weight of bentonite, and our preferred composition consists of 37% of bentonite, based on the dry weight of bentonite-sand mixture. Following the dry mixing of the bentonite with the sand, we add sufiicient water to the mixture to make a workable homogeneous mass. This mass is used as the back-fill material.

As a further anticorrosive feature, we may incorporate with the bentonite-sand mixture an amount of sodium sulfite, but our back-fill composition does not require the use of sulfite. It is desirable, however, because the sulfite removes the oxygen initially present in the bentonite-sand mixture. We incorporate from 0.01% up to 0.1% sodium sulfite to remove this oxygen.

Accordingly, it becomes a primary object of this invention to provide an anti-corrosive back-fill for buried metallic structures such as pipelines.

Another object of this invention is to provide an anticorrosive back-fill comprising about 1% to 10% of finelydivided, expanding-lattice clay, and sand having a certain particle size.

An object of this invention is to provide an anticorrosive back-fill containing 1% to 10% of an expanding-lattice clay, the balance being sand of a critical particle size.

Another object of this invention is to provide an anticorrosive back-fill composition containing:

In gredient Approx. percent by wt. 40-50 mesh silica sand 76 l40-170 mesh silica sand 11 200 mesh or greater flower of silica 12 200 mesh bentonite 1 Still another object of this invention is to provide an anticorrosive back-fill composition containing:

Ingredient Approx. percent by Wt.

Sand containing all particle sizes that pass through 40 mesh, including fines 97 200 mesh bentonite 3 These and other objects will be described orbecome apparent as the description proceeds.

A particularly effective application of the composition of this invention is in the field of seats for metallic structures such as storage tanks, refinery equipment, and the like, where there is a severe corrosion problem and the problem of supporting a large mass of weight per unit area under changing moisture conditions. The compositions of this invention'have been found 'to be particularly effective to overcome this dual problem.

T 0 illustrate, in constructing the seat for a fifty-footdiameter, oil-storage tank an excavation about 51 feet in diameter and 1 foot deep is made in the ground and properly leveled. A layer of crushed stone 3. inches thick is laid evenly in the bottom of the excavation followed by a 3" layer of gravel and a 3" layer of inert slag. These materials are laid and evenly spread to full depth so that upon compaction they meet the foregoing depth specifications.

If washed gravel, crushed stone, 'or slag is used, the aggregate in the lower course is graded 3 inches to 1% inches, and in the other courses 1 /2 inches to inch. Each course is rolled until maximum keying is obtained, after which dry screenings of limestone or slag are spread over the compacted surface and the surface broomed or vibrated until all the voids are filled. The course is next sprinkled and rolled with not less than a 104mm roller until the area is firmly compacted and waterbound.

Next, a cone-shaped sand cushion two inches thick at the periphery of the .tank and uniformly sloping upward to the center is placed in the prepared base, compacted,

and smoothed. This sand cushion is a homogenous mixture of 97% (by wt.) of sand containing all particle sizes (including fines) that will pass through a 40-mesh sieve and 3% of ZOO-mesh bentonite. The tank is then erected onthis base. tank is redressed and covered with coarse gravel, or an asphaltic concrete apron is placed as a seal thereover.

The clays intended for use in accordance with this invention are termed expanding-lattice clays and include the minerals known and used commercially under the name bentonite. This generic definition includes the various expanding-lattice minerals, such as mcntmorilloni-te, hectorite, saponite and nontronite, all of which have crystals that are flat and plate-like in structure and expand substantially in the presence of water. X-ray diffraction is used to measure the expanse properties of clays, and the useful clays are those in which the c-axis when saturated with water reaches a maximum greater than 25 Angstroms. Bentonite is known to be a natural hydrous silicate of alumina and contains also varying but small amounts of iron, lime, or magnesia, silicates, or alkalies. In the presence of water, bentonite forms a highly viscous solution or gel, which expands to many times the volume of the dry bentonite.

The alkali metal sulfites used as part of the compositions of this invention include sodium sulfite, lithium sulfite and cesiumsulfite. Sodium sulfite is prepared for economical reasons. The amount of alkali metal sulfite used may vary depending upon the amount of oxygen, particularly the dissolved oxygen present in the back-fill composition. Generally, this amount will be 0.01 to 0.1% by wt. of the total composition, taking into account the various amounts of dissolved oxygen that may be present in the moisture content of commercial sand and in the interstices or water content of expandinglattice clays. Generally, the use of about 0.05% by wt. of alkali metal sulfite is sufiicient to remove the dissolved oxygen in commercial sand or commercial clays.

The alkali metal sulfite is gradually oxidized to the alkali metal sulfate form as it consumes the oxygen present, and restores essentially non-oxidative conditions in the compositions as initially laid for a foundation. The invention contemplates the use of 0.01 to 0.1% by wt. of alkali metal sulfites in each of the compositions disclosed herein, and the foregoing specific examples may be read as including these amounts of alkali metal sulfite, particularly sodium sulfite. Other non-limiting example compositions would contain 98.9% sand as herein defined, 1% bentonite or other expanding-lattice clays, and 0.1% by wt. of sodium sulfite; or 98% sand as herein defined, 1.9% bentonite or other clay material, and 0.1% potassium sulfite.

No particular technique is necessary in compounding the ingredients of this invention into an operable backfill. quately mixed so that the clay and/or alkali metal sulfite is uniformly dispersed throughout the sand. gredients may be mixed by the use of known mechanical mixers or by hand. The compositions of this invention may be prepared at the site of the foundation, or separately and applied to the earth surface or in a cavity therein by known methods. For small foundations, handmixing is most economical. In the case of large installa- The exposed /2ft.-wide area outside the The only necessity is that the composition be ade- 1 The in-' tions, the use of scrapers, revolving mixers, and poweroperated blades may be used to either mix the composition in a pile or in a container, and thereafter apply it to the prepared earth surface, or mix the composition on the site of the foundation. In one embodiment of the invention, the sand portion of the composition may be spread over the foundation site and the expanding-lattice clay, with or Without the alkali metal sulfite already mixed therein, may be evenly spread over the surface of the sand and thereafter mechanically mixed with the sand.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. The method of preventing the corrosion of a steel structure to be supported in earth which comprises forming a supporting layer of a corrosion-resistant moisture barrier between the earth and said steel structure, said barrier comprising sand about 0.01 to 0.1% by weight of alkali metal sulfite and about 1% to 10% by Weight of bentonite, said sand having particle sizes all of which pass through a 40-mesh sieve.

2. The method in accordance with claim 1 in which said barrier comprises the following ingredients Ingredient-- Percent by wt. 40-50 mesh silica sand About 76 mesh silica sand About 11 200 mesh or greater flowers of silica About 12 200 mesh bentonite About 1 Alkali metal sulfite About 0.05

3. The method in accordance with claim 1 in which said barrier comprises the following ingredients Ingredient Percent by wt.

Sand containing all particle sizes that pass through 40 mesh, including fines About 97 200 mesh bentonite About 3 Alkali metal sulfite About 0.05

4. The method in accordance with claim 1 in which said alkali metal sulfite is sodium sulfite.

References Cited by the Examiner UNITED STATES PATENTS 393,477 11/88 Lake 61-1 842,201 1/07 Howe 947 1,348,278 8/20 Glynn 94--'2.5 X 1,761,165 6/30 Winch 947 2,007,969 7/35 Grodsky 61-72.1 2,277,286 3/42 Bechtner 50-100 X 2,343,764 3/44 Fuller 189-1 OTHER REFERENCES EARL J. WITMER, Primary Examiner.

JACOB L. NACKENOFF, Examiner. 

1. THE METHOD OF PREVENTING THE CORROSION OF A STEEL STRUCTURE TO BE SUPPORTED IN EARTH WHICH COMPRISES FORMING A SUPPORTING LAYER OF A CORROSION-RESISTANT MOISTURE BARRIER BETWEEN THE EARTH AND SAID STEEL STRUCTURE, SAID BARRIER COMPRISING SAND ABOUT 0.01 TO 0.1% BY WEIGHT OF ALKALI METAL SULFITE AND ABOUT 1% TO 10% BY WEIGHT OF BENTONITE, SAID SAND HAVING PARTICLE SIZES ALL OF WHICH PASS THROUGH A 40-MESH SIEVE. 